It is quite attractive to configure a photodetector with a monolithic integrated circuit using a silicon electronic technique in terms of cost and yield. A silicon-germanium optical receiver, that is, a silicon-germanium photodiode, which is configured with a monolithic integrated circuit on the same chip as a CMOS circuit is an attractive substitute for a hybrid optical receiver such as an InGaAs photodiode connected to a CMOS circuit or a GaAs circuit. An optical receiver configured with a monolithic integrated circuit can be manufactured using a standard silicon forming process, and thus is expected to be manufactured at a lower cost than a hybrid optical receiver.
A photodiode has been widely used as a device that converts an optical signal into an electrical signal quickly. A pin-type photodiode is a representative photodiode. A pin-type photodiode has a structure in which an i layer formed of an intrinsic semiconductor is interposed between a p layer made of a p-type semiconductor and an n layer formed of an n-type semiconductor. When a reverse bias voltage is applied from a bias supply to the stacked structure, almost all regions of the high-resistance i layer become a depletion layer of charge carriers. Photons of incident light are mainly absorbed into the i layer to generate electron-hole pairs. The generated electrons and holes drift in opposite directions in the depletion layer due to the reverse bias voltage to generate an electric current, and are detected as a signal voltage by a load resistor. Main factors restricting a response speed of photoelectric conversion are a circuit time constant decided by the product of capacitance generated by the load resistor and the depletion layer and a carrier-transit time required for electrons and holes to pass through the depletion layer.
As a photodiode having a short carrier-transit time, there is a Schottky-type photodiode. A Schottky-type photodiode has a structure in which a semi-transparent metallic film comes into contact with an n layer or an n− layer of a semiconductor. A Schottky barrier is formed near an interface in which the semi-transparent metallic film comes into contact with the n layer or the n− layer. Near the Schottky barrier, electrons of the n layer or the n− layer are diffused from the semi-transparent metallic film and become the depletion layer. In this state, when incident light is radiated, electrons are generated in the n layer or the n− layer, and drift in the depletion layer due to the reverse bias voltage. Further, as electrons are generated, light absorption in the element surface layer can be effectively used.
In the pin-type photodiode, for absorption of photons, the i layer, that is, the depletion layer, needs to have a sufficient thickness, but in the Schottky-type photodiode, the thickness of the depletion layer can be reduced. Thus, as described above, the carrier-transit time can be shortened using the Schottky-type photodiode. Further, as disclosed in Non-Patent Document 1, in the pin-type photodiode, in order to reduce the thickness of the depletion layer, an attempt to reduce an electrode interval has been performed using a lateral electrode structure, but light absorption efficiency in the semiconductor surface layer is bad, and it is difficult to achieve high sensitivity even when high speed operation is achieved.
Meanwhile, in the pin-type photodiode and Schottky-type photodiode, when an additional resistance value is reduced in order to decrease the circuit time constant, the voltage of a regenerative signal to be extracted decreases. Thus, a reduction in the capacitance of the depletion layer is important since it improves the SN ratio of the regenerative signal and reduces the reading error. Particularly, when the thickness of the depletion layer is reduced in order to shorten the carrier-transit time, the capacitance increases, and thus the area of the depletion layer or the Schottky junction needs to be decreased in order to increase the speed. However, when the area of the Schottky junction is decreased, utilization efficiency of signal light decreases, and there is consequently a problem in that the SN ratio of the regenerative signal deteriorates.
In order to solve the above problems, a metallic-semiconductor-metallic (MSM)-type photodiode in which two electrodes are periodically arranged on the same plane of a semiconductor has been proposed. In the MSM-type photodiode, an effective opposing area between the two electrodes is small, and a photodiode having small capacitance can be implemented. Further, since an electric current flows from one of the electrodes to the Schottky barrier in the forward direction, the additional resistance can be reduced to be smaller than in the pin-type photodiode, and a photodiode having a small circuit time constant can be implemented.
In the MSM-type photodiode, as described above, due to a voltage applied between two adjacent electrodes, a carrier depletion layer is formed, and photo carriers are swept to the electrode by an internal electric field.
Thus, in order to increase the speed, it is important to reduce an interval between two adjacent electrodes, reduce the thickness of the semiconductor light absorbing layer, and reduce the transit time of photo carriers.